1. Field of the Invention
The present invention relates to a control apparatus, a control method, and an engine control unit which control an output of a controlled object to converge to a target value in accordance with a deviation of the output from the target value.
2. Description of the Prior Art
Conventionally, a control apparatus of the type mentioned above is known, for example, from Laid-open Japanese Patent Application No. 2000-179385. Specifically, Laid-open Japanese Patent Application No. 2000-179385 describes an air/fuel ratio control apparatus for an internal combustion engine for controlling an air/fuel ratio of exhaust gases in an exhaust pipe of the internal combustion engine. An LAF sensor and an O2 sensor are provided respectively at a location upstream and at a location downstream of a catalyzer arranged in an exhaust passage of the internal combustion engine. The LAF sensor linearly detects an oxygen concentration in exhaust gases in a wide range of air/fuel ratio from a rich region to a lean region to output a detection signal KACT which is proportional to the detected oxygen concentration. The O2 sensor in turn generates a detection output VO2OUT at high level (for example, 0.8 volts) when an air/fuel mixture is richer than the stoichiometric air/fuel ratio; at low level (for example, 0.2 volts) when the air/fuel mixture is lean; and at a predetermined target value VO2TARGET (for example, 0.6 volts) between the high level and low level when the air/fuel mixture is near the stoichiometric air/fuel ratio.
The foregoing air/fuel ratio control apparatus relies on the following air/fuel ratio control to converge the air/fuel ratio of exhaust gases emitted from the internal combustion engine to a target value. First, the control apparatus calculates a basic fuel injection amount Tim and a correction coefficient KTOTAL therefor based on an operating condition of the internal combustion engine. Next, the control apparatus determines whether or not the internal combustion engine is in a predetermined operation mode in which the control apparatus should employ a target air/fuel ratio KCMD which is calculated by an adaptive sliding mode control different from this air/fuel ratio control. In this event, the control apparatus determines that the internal combustion engine is in the predetermined operation mode when the O2 sensor and LAF sensor are activated, and when an engine rotational speed NE and absolute intake pipe inner pressure PBA are within respective predetermined ranges. When the internal combustion engine is in the predetermined operation mode, as determined, the control apparatus reads the target air/fuel ratio KCMD calculated by the adaptive sliding mode control.
On the other hand, when the internal combustion engine is not in the predetermined operation mode, the control apparatus searches a map based on the engine rotational speed NE and absolute intake pipe inner pressure PBA to calculate the target air/fuel ratio KCMD. Next, the control apparatus calculates a variety of feedback coefficients #nKLAF, KFB. Then, the control apparatus corrects the target air/fuel ratio KCMD thus calculated in accordance with an air density to calculate a corrected target air/fuel ratio KCMDM. The control apparatus multiplies the basic fuel injection amount Tim by the total correction coefficient KTOTAL, corrected target air/fuel ratio KCMDM, and feedback coefficients #nKLAF, KFB to calculate a fuel injection amount #nTOUT for each cylinder and correct the resulting fuel injection amount #nTOUT for sticking. Subsequently, the control apparatus outputs a driving signal based on the fuel injection amount #nTOUT corrected for sticking to an fuel injector.
In the foregoing manner, the air/fuel ratio control apparatus controls the output KACT of the LAF sensor to converge to the target air/fuel ratio KCMD, and accordingly controls the output VO2OUT of the O2 sensor to converge to the target value VO2TARGET. Particularly, when the internal combustion engine is in the predetermined operation mode, the control apparatus employs the adaptive sliding mode control to calculate the target air/fuel ratio KCMD, so that the output VO2OUT of the O2 sensor can be more rapidly converged to the target value VO2TARGET than when the internal combustion engine is not in the predetermined operation mode. In other words, the control apparatus accurately controls the air/fuel ratio of an air/fuel mixture for the internal combustion engine to come closer to the stoichiometric air/fuel ratio with a high responsibility. Generally, a catalyzer most effectively purifies HC, CO, and NOx when the air/fuel ratio of the air/fuel mixture lies near the stoichiometric air/fuel ratio, so that the air/fuel ratio control apparatus can provide a satisfactory exhaust gas characteristic.
The conventional air/fuel ratio control apparatus described above can advantageously control the air/fuel ratio with a high responsibility when the internal combustion engine is in the predetermined operation mode by employing the adaptive sliding mode control to calculate the target air/fuel ratio KCMD. However, when the control apparatus conducts the foregoing adaptive sliding mode control when the internal combustion engine is in an extremely low load operation mode such as an idle operation mode, a reduced exhaust gas volume, longer response delay and dead time of the O2 sensor in providing the output VO2OUT, and a reduced range of the air/fuel ratio in which a stable combustion state can be ensured for the internal combustion engine will cause a degradation in controllability of the output VO2OUT of the O2 sensor with respect to the target value VO2TARGET. As a result, the air/fuel ratio of the air/fuel mixture fluctuates about the stoichiometric air/fuel ratio to reduce the purification percentage of exhaust gases by the catalyzer, possibly resulting in exacerbated characteristic of exhaust gases purified by the catalyzer (hereinafter called the “post-catalyst exhaust gas characteristic”).